4.7 What are the characteristics of FS-based compost?

The characteristics of faecal sludge-based compost vary depending on the specific composting method employed and the nature of the source material, including sludge characteristics, the type of bulking material used and the mixing ratio).

In theory, faecal sludge-based compost tends to have unique nutrient characteristics, including higher concentrations of nitrogen and phosphorus compared to composts derived solely from plant-based materials. However, in practice, losses of nitrogen (ammonia) during pre-composting storage and treatment, such as dewatering on sludge drying beds, may occur, potentially diminishing the nutrient content in the final compost.

The table below illustrates the ranges for selected constituents in compost where human waste was one of the raw materials. The broad ranges indicate variations observed under different composting conditions. In some instances, faecal sludge was combined with household waste and straw, while in others, it was mixed with municipal solid waste.

Composts from plant-based sources typically have a lower pathogen content, as the materials themselves are less likely to carry human pathogens. The composting process of human waste requires adherence to specific conditions regarding temperatures and durations (see Topic 4.4 for more details). Properly managed composting can significantly reduce the presence of harmful bacteria, viruses and parasites.

In line with processing specifications (see Topic 4.5 for more details), the final compost should maintain a specific moisture content. A level exceeding 35% is preferred, as anything lower would categorize it as dry, resulting in a dusty and potentially irritating material to handle. Conversely, compost with elevated moisture concentrations tends to become dense, clumpy, and more challenging to transport. Therefore, the optimal moisture content for the final compost typically falls within the range of 40 to 60%.

Generally, faecal sludge-based compost may be subject to additional regulatory scrutiny due to the potential presence of human pathogens. We will learn more about the reuse of sludge and the requirements for its application in lesson 7.

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